1. Field of the Invention
The present invention relates to an image forming apparatus having a function to control a heating unit for use in a fixing heat roller or the like.
2. Description of the Related Art
Fixing heat rollers are used to fix unfixed toner on a sheet in an image forming apparatus such as a copying machine or a printer which performs an electrophotographic process to generate an image on the sheet. A pair of such heat rollers, which are heated by heaters at a predetermined temperature, are vertically arranged in order to transport a sheet which is being heated and pinched therebetween under pressure. The heaters are provided for the respective heat rollers, and set to a temperature at which fixation process progresses. In the case where a plurality of heaters are temperature controlled, thermistors are provided for the respective heaters.
In this case, since the power consumption of a heater is large, the respective heaters are controlled in order not to concurrently turn on as described in Japanese Patent Published Application No. Hei 2000-70056 and Japanese Patent Published Application No. Hei 2003-195680. Namely, the peak power level is suppressed by controlling the heating period T of the respective heaters in order to successively change energization from one heater to the other.
In the case where a plurality of heaters are not turned on at the same time as described above, while one heater is turned on, the other heater is turned off. Because of this, a heater being turned off may not maintain the fixing temperature at a constant level. In addition to this, the heating operation is inefficient because, for example, even if one heater reaches a target temperature, the energization cannot be changed over to the other heater since the respective switching periods are fixed.